The present invention is related to processes for the separation of water from a mixture of water with other components, comprising the following steps:    A) providing feed material FM comprising water and at least one a nonionic surfactant S in an amount of 0.1 to 1000 ppm by weight based on the feed material FM,    B) subjecting said feed material FM to a distillation step using a falling film evaporator.
The separation of water from aqueous mixtures or solutions is a process of utmost technical relevance.
In particular, thermal desalination of water is of enormous importance. Standard processes for the thermal desalination of water include multi effect distillation (MED) and multi-stage effect distillation (MSF). While these processes have been employed for a long time, these processes still require improvement with respect to their efficiency.
One embodiment of MED uses a horizontal tube falling film evaporator. The falling film evaporator with horizontal tube bundles exhibit relatively high heat transfer coefficients under clean surface condition. However, they are susceptible to heat transfer deterioration and scale formation accompanied by film breakdown.
One embodiment of MED uses a vertical tube falling film evaporator.
In one embodiment the feed material is supplied by spraying onto the evaporator tube bundles using nozzles. The supply of feed material must be enough to form water film that evenly wets the surface and flows in a stable manner over the tubes.
For a more energy efficient operation, it is preferable to operate the evaporator in a MED plant in the laminar flow regime. Thereunder, less amount of feed material is necessary to be distributed over the tubes or, in other words, laminar flow regime requires lower wetting rates. In the laminar flow regime, a high wetting rate will lead to an unwanted increase in the thickness of the falling water film due to too much water supply or water overfeeding which reduces the heat transfer coefficient and causes a waste of energy for pumping or flooding of the evaporator.
Therefore it is preferable to operate a MED plant in the laminar flow regime. However, at low wetting rate the tube surface is susceptible to the occurrence of dry patches due to feed material maldistribution. Dry patches on the heated tube surface transmit essentially no heat and thus reduce thermal performance of the evaporator. Moreover, scale formation preferably starts at the edges of the dry patches where high local solution concentrations occur in the very thin film.
In industrial practice it is known that all heated tubes have to be maintained in a fully wetted state, that dry patches should be avoided and that the wetting rate has an influence on scale formation.
One limiting factor on the efficiency of thermal desalination processes used today, like in many other aqueous processes, is the formation of scale.
Another factor is to ensure that the falling film is not interrupted.